Dell Brocade DCX, Configuration Manual

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496 BigIron RX Series Configuration Guide
53-1001810-01
Traffic reduction parameters and algorithm
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Traffic reduction parameters and algorithm
A rate limiting policy specifies two parameters: requested rate and maximum burst.
Requested rate
The requested rate is the maximum number of bits a port is allowed to receive during a one-second
interval. The rate of the traffic that matches the rate limiting policy will not exceed the requested
rate.
The requested rate represents a percentage of an interface's line rate (bandwidth), expressed in
bits per second (bps).
Requested Rate must be entered in multiples of 515,624 bps. If you enter a number that is not a
multiple of 515,624, the software adjusts the rate down to the lowest multiple of the number so
that the calculation of credits does not result in a remainder of a partial Credit. For example, if you
enter 600,000 bps, the value will be adjusted to 515,624 bps.
Maximum burst
Maximum burst provides a higher than requested rate to traffic that meet the rate limiting criteria.
When the traffic on the port is less than the specified requested rate, the rate limiting policy can
accumulate credits up to a maximum, as specified in the maximum burst value. The accumulated
credit allows traffic to pass through the port for a short period of time, at a rate higher than the
average rate. The time period is determined by the amount of credit accumulated and the rate of
traffic passing through the port.
The maximum burst rate cannot be smaller than 65536 bits
Actual rate
The device determines actual rate limiting rates through the use of proprietary formulas built into
the packet processor hardware. The resulting rate that is the closest to the requested rate. This
leads to variable rate limiting granularities for rate limiting rates. The lower the configured rate
limiting rate, the finer the granularity; the higher the configured rate limiting rate, the larger the rate
increments. For example, at lower rates, say from 20,345 to 40, 330 bps, the configurable rate
limiting rates can increment by 1 bps, but at higher rate limiting rates, say between 4 Gbps to
line-rate, the configurable rates increment in the hundreds of Mbps.
The CLI shows actual rate as requested rate.
Credits and credit total
Each rate limiting policy is assigned a class. A class uses the average rate and maximum allowed
burst in the rate limiting policy to calculate credits and credit totals.
Credit size is measured in bytes. A credit is a forwarding allowance for a rate-limited port, and is the
smallest number of bytes that can be allowed during a rate limiting interval. The minimum credit
size can be 1 byte.
During a rate limiting interval, a port can send or receive only as many bytes as the port has Credits
for. For example, if an inbound rate limiting policy results in a port receiving two credits per rate
limiting interval, the port can send or receive a maximum of 2 bytes of data during that interval.